1. Field of the Invention
The invention is in the field of Nuclear Magnetic Resonance testing equipment. In particular the invention is an apparatus for NMR testing for azimuthal imaging of formation properties in borehole drilling.
2. Description of the Related Art
A variety of techniques have been used in connection with wellbore drilling to determine the presence of and to estimate quantities of hydrocarbons (oil and gas) in earth formations surrounding the wellbore. These methods are designed to determine formation parameters (in this application called “parameters of interest”) including, among other things, porosity, fluid content and permeability of the rock formation. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbore has been drilled. Removing the drilling apparatus in order to log the wellbore can prove costly in terms of time and money. More recently, wellbores have been logged simultaneously with drilling of the wellbores, which is referred to as measurement-while-drilling (“MWD”) or logging-while-drilling (“LWD”). Measurements have also been made when tripping a drillstring out of a wellbore. This is called measurement-while-tripping (“MWT”).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation, and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as “T1”), and transverse relaxation time (generally referred to as “T2”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
NMR well logging instrument typically include a permanent magnet to induce a static magnetic field in the earth formations and a transmitting antenna, positioned near the magnet and shaped so that a pulse of radio frequency (“RF”) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the static magnetic field. After an RF pulse, voltages are induced in a receiving antenna by precessional rotation of nuclear spin axes of hydrogen or other nuclei about the static magnetic field. The precessional rotation occurs in an excitation region where the static magnetic field strength corresponds to the frequency of RF magnetic field. A sequence of RF pulses can be designed to manipulate the nuclear magnetization, so that different aspects of the NMR properties of the formation can be obtained.
For NMR well logging the most common sequence is the CPMG sequence that comprises one excitation pulse and a plurality of refocusing pulses. The region of interest for these NMR methods generally lies totally within the rock formation. However, the sensitive volume, as defined by the magnitude of the static magnetic field and the frequency of the RF magnetic field can lie within the borehole, thus producing erroneous signals. Due to differing geometries of boreholes, different methods of NMR logging have been devised. For a small axially symmetric borehole in which the probing device is centrally located, it is possible to obtain information from an axially symmetric region within the rock formation.
A problem of interest in NMR logging is that of obtaining azimuthal information about earth formations surrounding a borehole. U.S. Pat. No. 5,977,768 to Sezginer, et al. teaches the use of a segmented antenna for obtaining such information. The static magnetic field is produced by a pair of opposed magnets with magnetization parallel to the longitudinal axis of the tool. The region of examination is a toroidal zone around the borehole. By the use of segmented antennas, each antenna receives signals primarily from a quadrant. U.S. Pat. No. 6,255,817 to Poitzsch, et al. teaches a method for analysis of data from the Sezginer device. U.S. Pat. No. 6,326,784 to Ganesan, et al. discloses an arrangement in which gradient coils are used to suppress spin-echo signals for portions of the region of examination. As would be known to those versed in the art, the toroidal region defined by the opposed magnet configuration is generally smaller than that of a transverse-dipole magnet arrangement. This feature restricts the region from which signals are obtained and further lowers the signal level
An apparently unrelated problem arises with tools using a transverse dipole magnet configuration. An example of this is in a “side-looking” NMR tool that is sensitive to NMR excitation on one side of the tool and less sensitive to NMR excitation on the other side. The more sensitive side of the tool is typically pressed against the sidewall of a borehole adjacent a formation, thereby providing minimum separation between the NMR tool's RF field generating assembly and the formation volume of NMR investigation. The less sensitive side of the tool is thus exposed to the borehole. This operational NMR technique is most effective when the borehole diameter is much greater than the diameter of the NMR tool.
Typically, side-looking NMR tools set up static and RF magnetic field distributions in a particular relationship to achieve maximum NMR sensitivity on one side of the NMR tool. These conventional side looking NMR techniques are well known in the art, as taught in the following patents: U.S. Pat. No. 4,717,877 to Taicher, et al., U.S. Pat. No. 5,055,787 to Kleinberg, et al., U.S. Pat. No. 5,488,342 to Hanley, U.S. Pat. No. 5,646,528 to Hanley, and U.S. Pat. No. 6,0213,164 to Prammer, et al.
The Kleinberg '787 patent teaches a side-looking NMR tool which generates a static magnetic field which results in a sensitive volume on only the front side of the tool. The sensitive region in front of this tool generates a field having a substantially zero gradient, while the region behind this tool has a relatively large gradient field. Consequently, the volume of the sensitive NMR region in front of the tool is much larger and contributes more significantly to the composite NMR signal, than does the NMR region behind the tool. The '787 patent technique, however, is only practical when the sensitive volume in front of the tool is very close to the tool. This condition therefore limits the available depth of NMR investigation. The '787 tool design also requires a substantially zero gradient in the sensitive volume. Such a zero gradient is not always desirable, however, in NMR well logging, as a number of associated NMR techniques depend upon having a finite, known gradient within the NMR sensitive volume.
The Hanley '342 patent teaches a NMR tool technique which provides a homogeneous region localized in front of the tool. The '342 tool design overcomes the disadvantageous requirement of the sensitive volume being undesirably close to the NMR tool. However, it suffers because the sensitive volume is not elongated along the longitudinal axis of the NMR tool or bore hole axis, causing unacceptable errors due to motional effects.
Hanley '528 discloses another variation of the Jackson device in which a shield of electrically conductive material is positioned adjacent to and laterally offset from the set of electrical coils whereby the magnetic field generated by the RF antenna is asymmetrically offset from the axis of the magnets. The region of uniform static field remains a toroid, as in the Jackson device. The Hanley '528 device may be operated eccentrically within a large borehole with a reduction in the borehole signal. Both of the Hanley devices suffer from the drawback that the axial extent of the region of examination is small, so that they cannot be operated at high logging speeds.
There are several devices in which the problem of limited axial extent of the basic Jackson configuration of permanent magnets is addressed. U.S. Pat. No. 4,717,877 to Taicher, et al. teaches the use of elongated cylindrical permanent magnets in which the poles are on opposite curved faces of the magnet. The static field from such a magnet is like that of a dipole centered on the geometric axis of the elongated magnets and provides a region of examination that is elongated parallel to the borehole axis. The RF coil in the Taicher device is also a dipole antenna with its center coincident with the geometric axis of the magnet, thereby providing orthogonality of the static and magnetic field over a full 360° azimuth around the borehole.
U.S. Pat. No. 6,023,164 to Prammer discloses a variation of the Taicher patent in which the tool is operated eccentrically within the borehole. In the Prammer device, NMR logging probe is provided with a sleeve having a semi-circular RF shield covering one of the poles of the magnet. The shield blocks signals from one side of the probe. The probe is provided with elements that press the uncovered side of the probe to the sidewall of the borehole so that signals received by the uncovered side arise primarily from the formation.
For both the Prammer '164 and the Hanley '528 devices, in order to get the best attenuation in the field behind the probe while maintaining sensitivity in front of the probe, the shield should be positioned as far away from the front region as possible. The effectiveness of the shield is limited by the diameter of the tool. In the absence of a shield, the Prammer '164 and Hanley '528 tools have a circular sensitive region, so that use of either device in an eccentric manner would result in a large signal from the borehole fluid.
The passive RF shield is typically positioned as far as possible from the front region in order not to spoil NMR tool sensitivity in the desired region and as close as possible to the back region for maximum effectiveness. It can be seen therefore that the effectiveness of the passive shield will eventually be limited by the diameter of the tool. If we can not achieve sufficient attenuation with a shield inside the tool we will have to adopt one of the following undesirable options: use the large magnet to move the rear region further away; reduce the signal from the front region; or place a shield outside the tool. Thus, neither approach presents a practicable solution.
U.S. Pat. No. 6,348,792 to Beard, et al., the contents of which are fully incorporated herein by reference, introduces a configuration of a primary static magnet with a secondary shaping magnet. The shaping magnet is used to shape the static magnetic field to conform to the RF field over a larger azimuthal sector around the tool. A shield in the back part of the device reduces the RF field behind the tool. The static and RF dipoles are rotated 90° relative to prior art, so that the static dipole points to the side of the tool and the RF dipole to the front of the tool. With this arrangement, eddy currents in the shield are substantially increased, increasing its effectiveness. U.S. Pat. No. 6,445,180 of Reiderman, et al., having the same assignee and the contents of which are fully incorporated herein by reference, teaches the use of a primary and secondary antenna system with the tool of the Beard patent. The primary antenna, being the larger of the two, creates a volumetrically extended magnetic field, most of which extends into the rock formation, and some of which lies within the borehole. The secondary antenna acts synchronously with the primary antenna, but its current circulates in a direction opposite to the direction of the current in the primary antenna, causing a magnetic field that cancels the magnetic field of the primary antenna in the region inside the borehole, thereby significantly reducing contributions from the borehole to the sensed NMR signal.
A limitation of these particular applications is that the device has only a side-looking mode, which is useful for large boreholes. However, for small boreholes, it is advantageous to use a central mode which excites signals on all sides of the NMR tool. Logging of boreholes with different diameters would thus require the use of different tools and an associated increase in costs due to having a larger inventory of tools. U.S. Pat. No. 6,525,535 of Reiderman, et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method and device similar to that in the Reiderman '180 patent in which the secondary antenna may be used as a booster antenna in small boreholes. This makes it possible to use the same logging tool for a variety of borehole sizes.
However, when the borehole is very large, the device of the Reiderman '451 application may not be able to fully suppress signals from the borehole. This situation is illustrated in FIGS. 3a and 3b. The FIG. 3a shows a logging tool 311 disposed in a borehole 301. The tool is shown in the side-looking mode and the region of examination is denoted by the combination of 321, 323a and 323b. By use of the hardware compensation (which may include a spoiler antenna and the arrangement of the basic magnet and antenna configuration, none of which are shown in the figure), signals from the region 325 within the borehole are suppressed.
FIG. 3b shows the same logging tool 311 in a much larger borehole 301. As can be seen, a portion of the region of examination denoted by 323a and 323b now lies within the borehole. The borehole fluid includes a large quantity of water, so that the signal from the borehole fluid could be much larger than those from the formation. A similar problem occurs even in smaller boreholes with a large amount of washout. It would therefore be desirable to suppress signals from within the borehole using a method other than hardware compensation: this would make it possible to use the same logging tool in a much larger range of borehole sizes. This suppression of signals from a selected azimuthal sector is, in principle, the same problem discussed above with respect to azimuthal imaging of the formation. The present invention is directed towards a solution of this problem.